1. Field of the Invention
The present invention relates to a process for producing cubic system boron nitride. More particularly, it relates to a process for producing a cubic system boron nitride having high strength and high purity by converting hexagonal system boron nitride into cubic system boron nitride with calcium or strontium boron nitride which dissolves boron nitride.
2. Description of the Prior Arts
The cubic system boron nitride is a highly hard crystal having diamond like characteristics whose thermal and chemical characteristics are superior to those of diamond. Accordingly, the cubic system boron nitride has been considered to be important as an abrasive for quenched steel.
Heretofore, the cubic system boron nitride has been produced by converting hexagonal system boron nitride with a catalyst such as Ia, IIa or IIIa group element in the periodic table; and their nitrides, lead, tin, antimony; aluminum alloy e.g. Al-Fe, Al-Ni, Al-Co, Al-Si, Al-Mn or Al-Cr; urea or ammonium salts. However, when said metal catalyst is used as the catalyst, unstable borides and free boron are formed as by-products and the contamination of the cubic system boron nitride crystal with boron is found and the crystal is black and has relatively low strength.
When the metal nitride is used as the catalyst, the unreacted nitride remained in the lattice of the cubic system boron nitride and the cubic system boron nitride having high purity can not be obtained.
When urea or the ammonium salt is used, the resulting crystal has remarkably small as a particle diameter of 0.1 to 0.5 .mu.m.